Meta‐analysis of salt marsh vegetation impacts and recovery: a synthesis following the Deepwater Horizon oil spill

Abstract Marine oil spills continue to be a global issue, heightened by spill events such as the 2010 Deepwater Horizon spill in the Gulf of Mexico, the largest marine oil spill in US waters and among the largest worldwide, affecting over 1,000 km of sensitive wetland shorelines, primarily salt marshes supporting numerous ecosystem functions. To synthesize the effects of the oil spill on foundational vegetation species in the salt marsh ecosystem, Spartina alterniflora and Juncus roemerianus, we performed a meta‐analysis using data from 10 studies and 255 sampling sites over seven years post‐spill. We examined the hypotheses that the oil spill reduced plant cover, stem density, vegetation height, aboveground biomass, and belowground biomass, and tracked the degree of effects temporally to estimate recovery time frames. All plant metrics indicated impacts from oiling, with 20–100% maximum reductions depending on oiling level and marsh zone. Peak reductions of ~70–90% in total plant cover, total aboveground biomass, and belowground biomass were observed for heavily oiled sites at the marsh edge. Both Spartina and Juncus were impacted, with Juncus affected to a greater degree. Most plant metrics had recovery time frames of three years or longer, including multiple metrics with incomplete recovery over the duration of our data, at least seven years post‐spill. Belowground biomass was particularly concerning, because it declined over time in contrast with recovery trends in most aboveground metrics, serving as a strong indicator of ongoing impact, limited recovery, and impaired resilience. We conclude that the Deepwater Horizon spill had multiyear impacts on salt marsh vegetation, with full recovery likely to exceed 10 years, particularly in heavily oiled marshes, where erosion may preclude full recovery. Vegetation impacts and delayed recovery is likely to have exerted substantial influences on ecosystem processes and associated species, especially along heavily oiled shorelines. Our synthesis affords a greater understanding of ecosystem impacts and recovery following the Deepwater Horizon oil spill, and informs environmental impact analysis, contingency planning, emergency response, damage assessment, and restoration efforts related to oil spills.


Introduction
Analyses for lightly oiled and moderately oiled sites are provided below, similar in format to the analyses for the "all oiled" sites and heavily oiled sites presented in the main body of the paper. Studies with groups of sites reliably classified as lightly oiled (one study) and moderately oiled (three studies) were limited in number and duration of data. These analyses are provided with limited interpretation due to small sample size. These data are also included in the "all oiled" sites analyses in the main body of the paper (used in combination with other data). If comparing figures across oiling levels in the main body of the paper with the supplemental figures, be aware of differences in scaling of the x and y axes in some instances.

Plant Cover
Impacts to plant percent cover were qualitatively observed for the moderately oiled sites, with degrees of impact and recovery patterns somewhat similar to the all oiled sites, to a lesser degree in some instances, but including the absence of full recovery through the duration of data in most cases, two to three years post-spill (Appendix S2: Figure S1, Table S1). Plant cover was similarly impacted in the lightly oiled sites in the marsh interior, but not at the marsh edge (Appendix S2: Figure S1, Table S1).

Stem Density
Impacts to stem density were qualitatively observed in some cases for the moderately oiled sites, with degrees of impact and patterns of recovery similar to the all oiled sites, although Juncus roemerianus stem density at the marsh edge recovered more quickly (Appendix S2: Figure S2, Table S1). In the lightly oiled sites, Spartina alterniflora stem density indicated a possible qualitative impact and recovery trend in the marsh interior, but stem density was otherwise not impacted (Appendix S2: Figure S2, Table S1).

Vegetation Height
Impacts on vegetation height were qualitatively observed for the moderately oiled sites, with degrees of impact and patterns of recovery similar to the all oiled sites, including the absence of full recovery through the duration of our data, three years post-spill (Appendix S2: Figure S3, Table S1). Vegetation height did not appear to be impacted in the lightly oiled sites (Appendix S2: Figure S3, Table S1).

Aboveground Biomass
Impacts to aboveground biomass were qualitatively observed for the moderately oiled sites, with degrees of impact and patterns of recovery similar to the all oiled sites, but with recovery occurring more quickly in some cases (Appendix S2: Figure S4, Table S1).
Aboveground biomass was similarly impacted in the lightly oiled sites in the marsh interior, but not at the marsh edge (Appendix S2: Figure S4, Table S1).

Belowground Biomass
Impacts to belowground biomass were quantitatively observed at the marsh edge for the moderately oiled sites (p = 0.0026; B-H adjusted p = 0.0980), and qualitatively observed for the marsh interior, with degrees of impact similar to the all oiled sites, including the absence of full recovery through the duration of our data, four years post-spill (Appendix S2: Figure S5, Table   S1). Belowground biomass was not impacted in the lightly oiled sites (Appendix S2: Figure S5, Table S1).

Summary
Although the data were limited and statistical comparisons less compelling in most instances, impacts and recovery in the moderately oiled sites were often similar to the all oiled sites, although in a few cases the degree of impacts were lesser and recovery quicker in the 4 moderately oiled sites. Regardless, full vegetation recovery in the moderately oiled sites was not complete over the duration of our data including, importantly, for belowground biomass at the marsh edge. Impacts to the lightly oiled sites were also observed, but to a lesser degree and across fewer metrics, as might be expected. In contrast to sites with higher oiling levels, impacts to lightly oiled sites appeared to be more common in the marsh interior, possibly due to higher oiling levels for some sites in the marsh interior (see Hester et al. 2016, their